1. Field of the Invention
The present invention relates to an oxygen isotope separation system and a method therefor. More specifically, the invention relates to a newly invented pressure-driven AGMD (Air Gap Membrane Distillation) system applied to a multi-stage membrane distillation cells which can produce an oxygen isotope effectively and economically, and a method therefor.
2. Description of the Related Art
18O-enriched water (>90%) is used as a target in the cyclotron for the production of the β-emitting radioisotope 18F (half-life=109.7 min), which is essential for PET (Positron Emission Tomography) pharmaceutical [18F]-labeled 2-deoxyglucose (FDG) synthesis. Demand for O-18 stable isotope increases as the superior tumor diagnostic feasibility obtained from PET increases. Economic mass production of the stable oxygen isotope, hence, is important and necessary.
As the separation methods of the oxygen isotopes (99.86% for O-16, 0.04% for O-17, and 0.2% for O-18 in nature), thermal diffusion, electrolysis, chemical exchange, gas diffusion, laser isotope separation, fractional distillation, cold distillation, and membrane distillation have been developed. However, fractional distillation of water is mainly used in the current production system while membrane distillation has been investigated to increase its applicability to the real production system since early 1990's due to its high separation factors compared to the other processes.
Distillation, which uses the different boiling point (same as the equilibrium vapor pressure) of the given materials, is known as the most effective process for separation of the light isotopes such as hydrogen, carbon, and oxygen, etc. Membrane distillation uses the equilibrium vapor pressure of the materials as well as their differentiated pore diffusion characteristics, while fractional distillation and cold distillation use only the vapor pressure difference of the materials.
Cold distillation for separation of the oxygen stable isotope uses the equilibrium vapor pressure differences between isotopic NO (Nitric Oxide) whose boiling point is very low. It is advantageous because its stage separation factor is relatively high (separation factor for O-18, α˜1.03) and also it can produce the other isotopes, N-14 and N-15, as byproducts. However, the separation system should be built by the resistive materials due to corrosive nature of NO and also it should be handled carefully due to its toxicity. It is also required to use thermal insulation to operate the system long period since the process requires the cold temperature as low as 77 K. More importantly, it is not economic because the additional material conversion system is necessary to transfer the initial material nitric acid to water which is used in the cyclotron as a target.
Fractional distillation of water, however, is more efficient than cold distillation because capital and operation costs are lower than cold distillation since no heavy thermal insulation is necessary due to its relatively high process temperature (330K). In addition, the product of the process is directly used in a cyclotron to produce the radioisotope F-18. Although fractional distillation has many advantageous merits compared to cold distillation, it is still an expensive process due to its huge distillation towers and long equilibrium time caused by relatively low separation factor (α˜1.0037).
On the other hand, membrane distillation as a substitute of the current production process has been explored to apply it to real production system since it is introduced first in U.S. Pat. No. 5,057,225. The separation factor for membrane distillation is much higher, α˜1.01˜1.04, than fractional distillation while it is competitive to cold distillation, since it uses the equilibrium vapor pressure effect, the same with the other distillation process, as well as differentiated diffusion characteristics of the particles with different masses in the membrane pores.
FIG. 1 shows the various membrane distillation methods currently developed. FIG. 1(a) shows Air Gap Membrane Distillation (AGMD) which constitutes with three parts, membrane upper part for water flow-in (hereafter feed) and flow-out (hereafter product); mid part for the water vapor permeate (hereafter tail); and lower part for cold water flowing. When hot water feed flows through on the membrane, water vapor is produced on the membrane surface based on the water temperature. The concentrations of the oxygen isotopic water molecules in the feed water and the water vapor are different, i.e. the concentration of the heavy molecule H218O in liquid water (product) is higher than in the water vapor, and vice versa for H218O under the certain temperature condition. In addition to the effect of equilibrium vapor pressure, the tail (membrane permeated water vapor) contains higher concentration of lighter water molecules than in the water vapor on the membrane surface due to its faster diffusion than the heavy molecules in the membrane pores. As a result of combined effects of vapor pressure and diffusion, the product contains heavier water molecules while the tail contains lighter water molecules. To promote the vapor permeation, temperature gradient producing driving force to the membrane interface was applied by a heat exchange plate (or permeation water vapor collector) which is cooled by a flowing cold fluid through the lower part of the permeation cell.
FIG. 1(b) indicates Vacuum Enhanced Membrane Distillation (VEMD). It is distinguished from the other processes by a high permeation flux generated from vacuum pump driven pressure reduction in the lower part of the membrane.
FIG. 1(c) shows the Direct Contact Membrane Distillation (DCMD). It is different from AGMD in the point of view that the tail of DCMD is mixed with a cooling fluid.
FIG. (d) demonstrates Sweep Gas Membrane Distillation (SGMD). In this process, the tail is mixed and collected by a sweep gas flowing though the lower part of the membrane.
Above mentioned membrane distillation processes produce the separation factors in between 1.01˜1.04 dependent on the experimental conditions. These factors are much higher than those of fraction distillation which uses the water as a feed same with membrane distillation. Since the concentration of H218O produced from a permeation cell is still too low even with the relatively high separation factors, it is required to build a multi-stage membrane distillation system to obtain highly enriched H218O.
Hence, it is important to iterate the MD processes shown in FIG. 1 and to optimize the operational conditions such as feed flow rate and feed temperature. First of all, the temperatures of the feeds at each stage must be maintained at the same under the given condition. Since the product produced from one stage must be supplied to the next stage as a feed, configuration of the stages should be optimized to push the feed through the stages.
For DCMD and SGMD, construction of a multi-stage system is not proper since the tailed water vapors in these processes are mixed with a cooling fluid or sweep gas. Every stage must have the tailed water vapor separation system to reflux those to the feed of the previous stages in the multi-stage system. This is too costly and complicated to build and operate the system.
It is not also proper for VEMD to construct a multi-stage system since the cold trap to collect the tailed permeated water vapor delivered by a vacuum pump is necessary. Also, the systems to evaporate and to reflux the water vapor are required in VEMD. For AGMD, it is not applicable to a multi-stage system directly since the separation factors are relatively low compared to the other MD processes and the tailed water vapor can not be delivered to the previous stage as a feed with current design and setup.
Therefore, MD is not applicable to construct a multi-stage system to produce highly enriched O-18 water so far.